1. Field of the Invention
The present invention relates to a method of detecting linear images in planar pictures, whereby linear images like that traced by a moving object, such as debris following an orbit, can be detected from a planar picture depicting an object, such as a night sky, or the like, captured by means of an imaging element, such as a CCD imaging element, for example.
2. Description of the Related Art
With the progress of space exploration in recent years, the number of rockets and artificial satellites launched has increased, and as a result, there is a large amount of space debris (space garbage) of various sizes, circling in the space surrounding the earth, including not only natural objects, such as space dust, but also artificial satellites, rockets and boosters which are no longer in use, or pieces or broken shards of same, or flakes of paint peeled off same, and the like.
In the use of celestial space, in addition to the unmanned artificial satellites orbiting the earth, there are other celestial objects in flight and orbit, such as manned spacecraft, space shuttles, space stations, and the like, and furthermore, journeys outside the spacecraft are also made by astronauts. This use of and activity in celestial space is carried out in a region of space traversed by large quantities of flying space debris, and as spacecraft and flying objects get larger in size and spend longer flight times in space, so the risk of these craft and objects, or astronauts performing a journey outside the craft, being struck by space debris gradually increases. Although space debris only has a size of several millimeters, for example, and collisions with same are extremely rare, if a collision does occur, the impact energy will be very high, due to the extremely fast relative velocity on impact, and hence the craft or object may be damaged and the lives of the astronauts put in danger. With increasing exploration and use of space in the future, it can be expected that the amount of space debris originating from artificial objects will rise further, and hence the danger of collision with this space debris will continue to increase.
Therefore, as one means of avoiding collisions with space debris, there have been demands for monitoring of space debris to be increased. By identifying the orbital path of individual items of space debris by performing monitoring, it is possible to predict space debris that has a possibility of coming close to an object or craft, and hence avoid collisions in advance.
Conventionally, space debris orbiting the earth has been tracked and monitored principally on the basis of earth-based observation by radar, or cameras attached to telescopes. In earth-based observation, a method for detecting linear images on a CCD picture is employed, and a method based on the Hough transform is generally used to detecting these linear images. In the Hough transform, it is necessary that the linear images appear clearly on the CCD picture, and therefore, it is not possible to provide satisfactory observation of space debris, which is small and dark and cannot be treated as a linear image on the picture.
In current international observation networks, any items 10 cm or above in size can be observed easily, and hence their orbits are being identified and catalogued, but any debris in the range of 1 cm–10 cm in size is not being observed or catalogued. Space debris of 1 cm or less in size will have relatively low impact energy in the event of a collision, and therefore protecting means, such as debris bumpers, or the like, provided on spacecraft, can be used against this debris, but any items of debris of 1 cm or larger will have too great an impact energy to be protected against by this means. It has been proposed that observation devices be installed in artificial satellites and spacecraft in order to monitor space debris which is 10 cm or less(for example, Japanese Patent Application Laid-open No. H5-286500, or Japanese Patent Application Laid-open No. 2000-190900). When launch expenses are taken into account, installing an observation device in a artificial satellite or spacecraft involves very great cost. Space exploration involving activities outside the craft is usually carried out from spacecraft circling in low altitude orbits, and therefore, environmental monitoring of debris several centimeters in size travelling in low orbit is absolutely essential in order to protect humans from collision with debris whilst performing space exploration activities in the future.
Other dark objects similar to the space debris from man-made objects include objects of natural origin, such as space dust and meteors. Images captured of these objects generated contain noise, which make the objects hard to detect.
Therefore, it is necessary to provide a way of identifying the position of a linear image representing the path of an object in a picture, by devising suitable image data processing of a planar picture, even in the case of a planar picture where linear images, such as the trails caused by movement of an object, do not appear clearly.